Whenever a defect occurs in a semiconductor device it is desirable to be able to locate the source of the problem, thereby allowing the problem to be addressed. Numerous techniques have therefore been developed for locating defects in a die in the x-y plane as shown in FIG. 1. In particular scanning techniques such as e-beam (electron beam) and o-beam (laser beam) scanning has been developed to locate the source of a defect. However, this leaves questions as to the depth (z-axis) at which the defect identified in the x-y plane, is located. One solution to this problem is the gradual grinding or lapping away of layers of the semiconductor die. This involves a device such as the one shown in FIG. 2 which comprises a lapping wheel 200. The die 202 to be analyzed is attached to a fixture (sample holder) 204, which comprises a steel supporting plate. Typically the die 202 is attached to the sample holder 204 by means of wax, and the sample holder 204 is in turn connected to an oscillating head 206. During the lapping process the head 206 is lowered so that the die 202 engages the wheel 200. The wheel 200 has an abrasive surface 210 and rotates about a vertical axis 212. As the wheel 200 rotates, the head 206 oscillates back and forth to move the die laterally back and forth between the central axis 212 and the periphery 214 of the wheel. One problem with the lapping device is that it tends to lap or grind away the die more quickly along the outer edge 216 of the die and especially the corners of the die due to the greater speed of the lapping wheel 200 towards its periphery 204 as will be discussed in greater detail below. FIGS. 3 and 4 show a typical die 202 secured to a sample holder 204 and show how the profile of the die 202 changes in the course of lapping. FIG. 3 shows the profile before lapping and FIG. 4 shows the profile after lapping. It will be appreciated from the profile in FIG. 4 that if the defect is located along the outer edge 216 it becomes difficult to control the lapping to avoid lapping away all or portion of the defective region. The present invention seeks to provide a way of improving the lapping profile in the edge and corner regions of a semiconductor die.